1. Field of the Invention
The present invention relates to a servo control method for an orbital machining with a cutting tool, and a servo control system for an orbital machining. More particularly, the invention relates to a servo control method for an orbital machining with a cutting tool using an NC (numerical control) machine tool having a simultaneous multiple-axes control function, and a servo control system for an orbital machining.
2. Description of Relevant Art
As a machining method for effectively performing, by way of a rotary cutting, a machining of a hole with an arbitrary inside diameter and a machining of an outer circumferential surface with an arbitrary outside diameter, as well as a taper machining, a spherical surface machining, a polygonal machining, a thread machining, a flange face machining, and a voluntary shape machining, by using a single cutting tool, irrespective of a radius (a tool diameter) of the cutting tool, there has been proposed in Japanese Patent Application Laid-Open Publication No. 8-126938 (in a family including Korean Patent Publication No. 180954 and U.S. Pat. No. 5,713,253) an orbital machining in which a spindle having a cutting tool attached thereto and a work to be machined are moved, by feed shaft control, to make a relative displacement to each other along a plane perpendicular to an axis of rotation of the spindle, such that a mutual interpolation motion is achieved between the spindle and the work, and a rotation angle of the spindle is controlled quantitatively and synchronously to have a predetermined correlation to the shaft control so that, at all angular positions in rotation of the spindle, a blade direction of the cutting tool is maintained in a preset direction to thereby achieve a cutting into a configuration to be defined by an interpolation locus based on the mutual interpolation motion.
For such an orbital machining, there is employed a general-purpose NC machine tool which has a simultaneous multiple-axes control function and in which a spindle and feed shafts are controlled by servo motors.
In the servo control, there are developed control follow-up delays in motor servo systems of the feed shafts in dependence on their positional gains. As a feed speed becomes higher, a corresponding control follow-up delay becomes significant, and causes an error to normally occur in a feed position, or a reduction of cutting radius to be accompanied in a boring, so that the machining does not comply with an associated command value.
To this point, it may be proposed to make a feed-forward compensation for a control follow-up delay in a motor servo system of a feed shaft by adding, to a positional command value of the feed shaft, a feed-forward compensation value determined by such a calculation that (a differential value of the positional command value)/(a position loop gain of the feed shaft motor servo system).
If feed-forward compensations are effected for control follow-up delays in motor servo systems of feed shafts, then loci of (interpolation) motions of a work relative to a spindle can be compensated to be complaint with command values. However, there still remains a control follow-up delay in a motor servo system for control of a rotation angle of a spindle, and there occurs such a situation that control follow-up delays of the motor servo systems of the feed shafts are eliminated by feed-forward compensations while a control follow-up delay is left in the motor servo system for rotation angle control of the spindle, whereby a synchronization collapses between feed shaft control and spindle rotation angle control, so that an angular motion in rotation of the spindle delays relative to a circular interpolation motion of the machine, with a result that a cut dimension has an error, or a reduction of cutting radius occurs in a boring, and a high-precision orbital machining is difficult.
Further, the machine tool has, in its mechanical system, errors due to elastic deformations caused by inertial forces of a system of feed shafts and a spindle, and such errors increase, as feed speeds of the shafts, a rotation speed of the spindle and/or their accelerations (or decelerations) become higher, whereby also the precision of orbital machining is lowered.